Microwave emission from plasmas in InSb with and without magnetic fields

A theory of wave propagation through semiconductors, assuming general orientations of the carrier drift velocities, the propagation vector, and the RF electric field relative to an applied steady magnetic field, is derived. This is specialized to obtain a theory of pseudolongitudinal wave interactions in electron and hole streams in InSb in a transverse and in a parallel (or zero) magnetic field. The theory explains the various features of high electric-field microwave emission and RF oscillations observed by experimenters over several years in a wide variety of material conditions and field configurations. Explained in particular are the facts that: 1) starting n- or p-type materials with initial density ≲ 10
¹⁴
cm
^-3
have to be impact ionized to achieve electron or hole number densities adequate for microwave emission; 2) for impact-ionized plasmas, it takes a minimum transverse magnetic field (
B\perp
) on the order of 1.5-2.5 kG for the onset of microwave emission and the threshold current thereafter diminishes monotonically with increasing
B\perp
; 3) p-type material with high enough initial density (n
h
≳ 1 - 3 × 10
¹⁵
cm
^-3
) is unstable with electron injecting contacts in the presence of 15-20 percent (or more) electron injection; and 4) in the absence of
B\perp
, a longitudinal magnetic field is necessary mainly to prevent pinching of the plasma at the high current densities needed for microwave emission, etc. RF oscillations observed in n- and p-type InSb with or without
B\perp
are also explained. The dependence of the oscillation frequency on the magnitude of
B\perp
shows a remarkable correlation with experimental observations.